Internet Engineering Task Force H. Stenn
Internet-Draft D. Mills
Obsoletes: 7822 (if approved) Network Time Foundation
Intended status: Standards Track October 2, 2018
Expires: April 5, 2019
Network Time Protocol Version 4 (NTPv4) Extension Fields
draft-stenn-ntp-extension-fields-08
Abstract
Network Time Protocol version 4 (NTPv4) defines the optional usage of
extension fields. An extension field, as defined in RFC 5905
[RFC5905] and RFC 5906 [RFC5906], resides after the end of the NTP
header and supplies optional capabilities or information that cannot
be conveyed in the basic NTP packet. This document updates RFC 5905
[RFC5905] by clarifying some points regarding NTP extension fields
and their usage with legacy Message Authentication Codes (MACs), and
removes wasteful requirements added by RCF 7822 [RFC7822].
This proposal deprecates RFC 7822 [RFC7822].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on April 5, 2019.
Copyright Notice
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document authors. All rights reserved.
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Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2.2. Terms and Abbreviations . . . . . . . . . . . . . . . . . 3
3. NTP MAC - RFC 5906 Update . . . . . . . . . . . . . . . . . . 4
3.1. RFC5906 Section 4. - Autokey Cryptography . . . . . . . . 4
3.2. RFC5906 Section 10. - Autokey Protocol Messages . . . . . 4
3.3. RFC5906 Section 11.5. - Error Recovery . . . . . . . . . 4
3.4. RFC5906 Section 13. - IANA Consideration . . . . . . . . 4
4. NTP Extension Fields - RFC 5905 Update . . . . . . . . . . . 5
4.1. OLD: 'RFC5905 7.5 - NTP Extension Field Format' . . . . . 5
4.2. NEW: 'RFC5905 Section 7.5 - NTP Extension Field Format' . 5
4.3. NEW: 'RFC5905 Section 7.5.1 - Extension Fields and MACs' 8
4.4. OLD: 'RFC5905 Section 9.2. - Peer Process Operations' . . 9
4.5. NEW: 'RFC5905 Section 9.2. - Peer Process Operations' . . 9
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. Normative References . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
An NTP packet consists of a set of fixed fields that may be followed
by optional fields. Two types of optional fields are defined:
extension fields (EFs) as defined in Section 7.5 of RFC 5905
[RFC5905], and legacy Message Authentication Codes (legacy MACs).
If a legacy MAC is used, it resides at the end of the packet. This
field can be either a 4-octet crypto-NAK or data that has
traditionally been 16, 20 or 24 octets long.
Additional information about the content of a MAC is specified in RFC
5906 [RFC5906], but since that RFC is Informational an implementor
that was not planning to provide Autokey would likely never read that
document. The result of this would be interoperability problems, at
least. To address this problem this proposal also copies and
clarifies some of the content of RFC 5906, putting it into RFC 5905.
Because there is a reasonable expectation that RFC 5906 will be
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deprecated, this document does not propose changes or updates to RFC
5906.
NTP extension fields are defined in RFC 5905 [RFC5905] as a generic
mechanism that allows the addition of future extensions and features
without modifying the NTP header format (Section 16 of RFC 5905
[RFC5905]).
With the knowledge and experience we have gained over time, it has
become clear that simplifications, clarifications, and improvements
can be made to the NTP specification around EFs and MACs.
This proposal adjusts and clarifies the requirements around EFs and
MACs, allows EFs to be on 4-octet boundaries of any acceptable
length, and provides methods to disambiguate packet parsing in the
unexpected and unlikely case where an implementation would choose to
send a packet that could be ambiguously parsed by the receiver.
This proposal deprecates RFC 7822 [RFC7822].
Implementations are still free to send EFs that are padded to longer
lengths that otherwise follow the requirements below.
This document better specifies and clarifies extension fields as well
as the requirements and parsing of a legacy MAC, with changes to
address errors found after the publication of RFC 5905 [RFC5905] with
respect to extension fields. Specifically, this document updates
Section 7.5 of RFC 5905 [RFC5905], clarifying the relationship
between extension fields and MACs, and expressly defines the behavior
of a host that receives an unknown extension field.
2. Conventions Used in This Document
2.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2.2. Terms and Abbreviations
EF - Extension Field
MAC - Message Authentication Code
NTPv4 - Network Time Protocol, Version 4 RFC 5905 [RFC5905]
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3. NTP MAC - RFC 5906 Update
This document copies and updates some information in RFC 5906
[RFC5906] and puts it in to RFC 5905, as follows:
3.1. RFC5906 Section 4. - Autokey Cryptography
This section describes some of the cryptography aspects of Autokey.
The third paragraph describes the use of 128- and 160-bit message
digests. The enumeration of 128- and 160-bit message digests is not
meant to be limiting - other message digest lengths MAY be
implemented. This paragraph also describes some of the expected
semantic ranges of the key ID. This information belongs in RFC 5905.
The key ID value is particularly significant because it provides
additional detection and disambiguation protection when deciding if
the next data portion is either a legacy MAC or an extension field.
[This is additional evidence that although RFC 5906 is Informational,
parts of its content are REQUIRED for proper behavior of RFC 5905.]
3.2. RFC5906 Section 10. - Autokey Protocol Messages
This section describes the extension field format, including initial
flag bits, a Code field, and 8-bit Field Type, and the 16-bit Length.
This proposal expands and clarifies this information and puts it into
RFC 5905.
This section says "The reference implementation discards any packet
with a field length of more than 1024 characters." but this is no
longer true.
3.3. RFC5906 Section 11.5. - Error Recovery
This section describes the crypto-NAK, which should be described in
RFC 5905. A crypto-NAK is used by RFC 5905 as well. [This is
additional evidence that even though RFC 5906 was Informational, some
of its content is REQUIRED for proper behavior for RFC 5095.]
3.4. RFC5906 Section 13. - IANA Consideration
This section lists the Autokey-related Extension Field Types,
including Flag Bits, Codes, and Field Types, which should be
described in RFC 5905, or perhaps in some other document. [This is
additional evidence that even though RFC 5906 is Informational, some
of its content is REQUIRED for proper behavior for RFC 5905.]
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4. NTP Extension Fields - RFC 5905 Update
This document updates Section 7.5 of RFC 5905 [RFC5905] as follows:
4.1. OLD: 'RFC5905 7.5 - NTP Extension Field Format'
In NTPv4, one or more extension fields can be inserted after the
header and before the MAC, which is always present when an extension
field is present. Other than defining the field format, this
document makes no use of the field contents. An extension field
contains a request or response message in the format shown in
Figure 14.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------------------------------+
| Field Type | Field Length |
+-------------------------------+-------------------------------+
. .
. Value .
. .
+-------------------------------+-------------------------------+
| Padding (as needed) |
+---------------------------------------------------------------+
Figure 14: Extension Field Format
All extension fields are zero-padded to a word (four octets)
boundary. The Field Type field is specific to the defined function
and is not elaborated here. While the minimum field length
containing required fields is four words (16 octets), a maximum field
length remains to be established.
The Length field is a 16-bit unsigned integer that indicates the
length of the entire extension field in octets, including the Padding
field.
4.2. NEW: 'RFC5905 Section 7.5 - NTP Extension Field Format'
In NTPv4, one or more extension fields can be inserted after the
header and before the possibly optional legacy MAC. A MAC SHOULD be
present when an extension field is present. A MAC is always present
in some form when NTP packets are authenticated. This MAC SHOULD be
either a legacy MAC or a MAC-EF. It MAY be both. Other than
defining the field format, this document makes no use of the field
contents. An extension field contains a request or response message
in the format shown in Figure 14.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------------------------------+
| Field Type | Field Length |
+-------------------------------+-------------------------------+
. .
. Value .
. .
+-------------------------------+-------------------------------+
| Padding (as needed) |
+---------------------------------------------------------------+
Figure 14: Extension Field Format
The four octets that comprise the Field Type and Field Length are
called the Extension Field Header. Octets beyond the Extension Field
Header are called the Extension Field Body, or the Extension Field
Payload. The EF Body (EF Payload) MAY be null in some cases.
All extension fields are zero-padded to a word (four octet) boundary.
The Field Type is specific to the defined functionality and detailed
information about the Field Type is not elaborated here. The minimum
size of an Extension Field is a 32-bit word (4 octets), and while the
maximum extension field size MUST be 65532 octets or less, an NTP
packet SHOULD NOT exceed the network MTU.
The Field Length is a 16-bit unsigned integer that indicates the
length of the entire extension field in octets, including any Padding
octets. The bottom two bits of the Field Length SHOULD be zero, and
the size of the extension field SHOULD end on a 32-bit (4 octet)
boundary. [RFC5905 Section 7.5 says "All extension fields are zero-
padded to a word (four octets) boundary." but does not use 'MUST'
language. Is it overkill to reiterate this requirement here? Should
we use SHOULD or MUST regarding the bottom two bits or the boundary
of the EF? It is possible, down the road, that we might find some
use for those bottom 2 bits, even if we require a 32-bit boundary on
the last octet of an EF.]
The Field Type contains the following sub-elements:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------------------------------+
|R|E| Code | Type | (Field Length) |
+-------------------------------+-------------------------------+
Extension Field Header Format
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Where the following Field Type flags are defined:
R: 0 for "Information/Query", 1 for a "Response"
E: 0 for "OK", 1 for an "Error". Unused, and will be deprecated.
[The 'R' flag is currently used by Autokey, and by the proposed I-DO
extension field. This flag is used after the packet is accepted.]
[The 'E' flag was proposed for use by Autokey, after the packet was
accepted. As it was never used and no other use-cases have been
identified, we are recommending this flag be deprecated at some point
in the future.]
[The EF Code subtype is currently used by RFC 5906, Autokey
[RFC5906], by the proposed Extended Information EF proposal, and is
expected to be used by the NTS Extension Field, at least.]
The Autokey EF currently uses the most Code values - 10 of them,
which equates to the least-significant 4 bits of the high-order
octet. It is possible that additional flag bits will be allocated;
in the past, the high-order 2 bits were reserved, and for a time two
additional bits were proposed. Make no assumptions about the unused
bits in this octet.
The EF Header and Body fields (the Flags, Code, Type, and Length, and
any Value or Padding) are specific to the defined functionality and
are not elaborated here; appropriate Field Type Flags, the EF Code,
and EF Type values are defined in an IANA registry, and the Length,
Value, and Padding values are defined by the document referred to by
the registry. If a host receives an extension field with an unknown
Field Type, the host SHOULD ignore the extension field and MAY drop
the packet altogether, depending on local policy.
The Length field is a 16-bit unsigned integer that indicates the
length of the entire extension field in octets, including any
Padding.
While the minimum field length of an EF that contains no value or
padding fields is one word (four octets), and the minimum field
length of an EF that contains required fields is two words (8
octets), the maximum field length MUST NOT be longer than 65532
octets due to the maximum size of the data represented by the Length
field, and SHOULD be small enough that the size of the NTP packet
received by the client does not exceed the smallest MTU between the
sender and the recipient. The bottom two bits of the Field Length
SHOULD be zero and the EF data SHOULD be aligned to a 32-bit (4
octet) boundary.
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4.3. NEW: 'RFC5905 Section 7.5.1 - Extension Fields and MACs'
With the inclusion of additional Extension Fields, there is now a
potential that a poorly-designed implementation would produce an
ambiguous parsing in the presence of a legacy MAC. What follows are
two possibly independent ways to prevent this situation from ever
happening.
Note well that to-date, there are only two defined Extension Field
Types: Autokey, defined by RFC 5906 [RFC5906], and the Experimental
UDP Checksum Complement in the Network Time Protocol, defined by RFC
7821 [RFC7821].
In spite of its known serious problems, Autokey is still in use by
some and is a legacy case that is easily supported. Old systems will
still work. An old system will still be able to open a properly-
configured Autokey association to a new system, a new system will
still be able to open a properly-configured Autokey association with
an old system, and two new systems will be able to open a properly-
configured Autokey association.
The UDP Checksum Complement extension field forbids the use of a
legacy MAC, so any packet that uses it CANNOT be using a legacy MAC.
[We could list the detailed and specific reasons why traffic using
this EF is immune to EF/legacy MAC problems, but I fear that would
just be confusing to most people.]
The first and best way to prevent ambiguous parsing is to use the
I-DO extension field.
By definition any NTP client or server that handles any other
Extension Fields is "new code" and can completely prevent ambiguity
by the initiating side sending a packet containing an I-DO extension
field followed by an optional MAC-EF followed by an optional legacy
MAC. The inclusion of any MAC would be dictated by the
authentication requirements of the association.
Note that NTP traffic works perfectly well without using any other
extension fields. Newer extension fields offer additional
capabilities, but these capabilities are not required for operation.
[Even in the case of NTS or SNT, we're talking about "new code" that
can be expected to be aware of issues with new extension fields an
legacy MACs.]
If the initiating side sends an I-DO packet and gets no response, it
operates as if the other side cannot handle new extension fields and
simply continues the association without sending any new extension
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fields. At any point in the future a packet can be sent with an I-DO
extension field to see if the other side will respond.
An NTP implementation that receives a packet with an I-DO extension
field may respond with a packet that may or may not contain an I-DO
Response. If it does not respond, the other side SHOULD assume that
the receiver does not understand new EFs. If it responds without
sending an I-DO Response extension field, the sending side knows it
should not send any new extension fields to this server. If the
system that receives an I-DO extension field responds with an I-DO
Response, it's telling the sender exactly what capabilities it is
currently willing to exchange.
The second way to prevent ambiguous parsing is to use the LAST-EF
extension field.
By definition, if I-DO is used and each side agrees to support LAST-
EF then LAST-EF will prevent any ambiguity.
If, however, I-DO is not used then one side can simply send a packet
with a LAST-EF. The LAST-EF extension field could be four-octet
extension field, it could be a 28 octet extension field, or some
other length that ends on a 32-bit boundary. If the other side
responds appropriately then all is well. If the other side does not
respond appropriately the sender should proceed without sending any
new extension fields.
Parties interested in additional reasons for and approaches to
understanding why there is no reason to be concerned about potential
ambiguities with new code that would use new extension fields and
legacy MACs can look at the the drafts that preceded this document.
4.4. OLD: 'RFC5905 Section 9.2. - Peer Process Operations'
...
FXMIT. ... This message includes the normal NTP header data shown in
Figure 8, but with a MAC consisting of four octets of zeros. ...
4.5. NEW: 'RFC5905 Section 9.2. - Peer Process Operations'
...
FXMIT. ... This message includes the normal NTP header data shown in
Figure 8, but with a MAC consisting of four octets of zeros. This
MAC can be a legacy MAC or a MAC-EF. If it's a MAC-EF, the crypto-
NAK MUST be the only MAC in the MAC-EF payload. ...
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5. Acknowledgements
The authors wish to acknowledge the contributions of Sam Weiler,
Danny Mayer, and Tal Mizrahi.
6. IANA Considerations
This memo requests IANA to update the NTP Extension Field Types table
in the NTP Parameters document as follows. The following is expected
to be a functional superset of the existing information:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---------------+---------------+
|R|E| Code | Type |
+-------------------------------+
NTP Extension Field Type Format
Where the following Field Type flags are defined:
R: 0 for "Information/Query", 1 for a "Response"
E: 0 for "OK", 1 for an "Error". Unused, and will be deprecated.
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+------------+----------------------------------------------+
| Field Type | Meaning |
+------------+----------------------------------------------+
| 0x0000 | crypto-NAK (with Field Length of 0) |
| 0x0000 | RESERVED: Permanently Unassigned |
| 0x0001 | RESERVED: Unassigned |
| 0x0002 | Autokey: No-Operation Request |
| 0x8002 | Autokey: No-Operation Response |
| 0x0102 | Autokey: Association Message Request |
| 0x8102 | Autokey: Association Message Response |
| 0x0202 | Autokey: Certificate Message Request |
| 0x8202 | Autokey: Certificate Message Response |
| 0x0302 | Autokey: Cookie Message Request |
| 0x8302 | Autokey: Cookie Message Response |
| 0x0402 | Autokey: Autokey Message Request |
| 0x8402 | Autokey: Autokey Message Response |
| 0x0502 | Autokey: Leapseconds Value Message Request |
| 0x8502 | Autokey: Leapseconds Value Message Response |
| 0x0602 | Autokey: Sign Message Request |
| 0x8602 | Autokey: Sign Message Response |
| 0x0702 | Autokey: IFF Identity Message Request |
| 0x8702 | Autokey: IFF Identity Message Response |
| 0x0802 | Autokey: GQ Identity Message Request |
| 0x8802 | Autokey: GQ Identity Message Response |
| 0x0902 | Autokey: MV Identity Message Request |
| 0x8902 | Autokey: MV Identity Message Response |
| 0x0005 | Checksum Complement |
| 0x2005 | Checksum Complement (deprecated flag 0x2000) |
+------------+----------------------------------------------+
Current Extension Fields
7. Security Considerations
Additional information TBD, as needed.
8. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
.
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[RFC5906] Haberman, B., Ed. and D. Mills, "Network Time Protocol
Version 4: Autokey Specification", RFC 5906,
DOI 10.17487/RFC5906, June 2010,
.
[RFC7821] Mizrahi, T., "UDP Checksum Complement in the Network Time
Protocol (NTP)", RFC 7821, DOI 10.17487/RFC7821, March
2016, .
[RFC7822] Mizrahi, T. and D. Mayer, "Network Time Protocol Version 4
(NTPv4) Extension Fields", RFC 7822, DOI 10.17487/RFC7822,
March 2016, .
Authors' Addresses
Harlan Stenn
Network Time Foundation
P.O. Box 918
Talent, OR 97540
US
Email: stenn@nwtime.org
David L. Mills
Network Time Foundation
P.O. Box 918
Talent, OR 97540
US
Email: mills@udel.edu
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